Heparin neutralization with platelet factor 4

ABSTRACT

Circulating heparin in a mammal may be neutralized without substantial depletion of platelets or leukocytes by administering to the mammal a heparin neutralizing amount of purified PF4 or rPF4 or a heparin neutralizing fragment thereof.

This is a continuation of application Ser. No. 07/480,873, filed Feb.16, 1990, now abandoned.

FIELD OF THE INVENTION

The invention relates to neutralization of heparin with Platelet Factor-4 (PF4).

BACKGROUND OF THE INVENTION

Applications of advancing medical technology, such as cardiopulmonarybypass, are associated with a variety of complications. Even in shortterm use, blood oxygenators produce sufficient activation of clottingpathways to require the use of heparin to inhibit blood coagulation.

An invariable complication of such surgical procedures is a hemorrhagicstate manifested by a prolonged bleeding time, causes of which includefailure to adequately neutralize heparin and the continuous stimulationof platelets, as manifested by a fall in platelet count, stimulation ofthromboxane synthesis and release of platelet granule constituents. (SeeColman. J. Anesthesiology vol. 66 (5). May. 1987. p. 595). Reversal ofheparin is required to restore normal coagulation status and reducepost-operative blood loss.

Protamine is an arginine rich polypeptide (32 amino acids from salmon)commonly used at the conclusion of cardiovascular surgical procedures toneutralize the anticlotting effects of heparin. The use of protamine,however, has been linked to several post-surgical complications, some ofwhich are postoperative systemic hypotension, allergic reactions,catastrophic pulmonary vasoconstriction, acute pulmonary hypertension,complement activation, noncardiogenic pulmonary edema, decreased cardiacoutput (later event). and thrombocytopenia/leukopenia.

The underlying biochemical basis for these physical complications ispoorly understood, but allergic reactions to protamine, have been welldocumented. Since protamine, usually isolated from fish, can berecognized as a foreign protein by the human immune system, patientswith prior protamine exposure are at particular risk during subsequentexposures (Just Viera, J.O., Amer. Surgeon 50, (1984), 151-163).Additionally, studies suggest that a non-immunological pathway viacomplement activation may be responsible for many of the acute reactionsobserved during protamine reversal of heparin anticoagulation.

To avoid the use of protamine, a number of approaches have beenproposed. Construction of bypass circuits with materials that do notactivate the coagulation cascade have been suggested, as well as the useof non-heparin anticoagulate preparations. Neutralizing agents forheparin other than protamine are also currently being sought. Horrow,"Effective Hemostasis in Cardiac Surgery", chap. 2, Ellison and Jobes,Eds, in press, 1988. All of these alternatives, presently in variousstages of research, have yet to reveal a suitable substitute forprotamine that has gained widespread acceptance.

PF4 is a well characterized heparin binding protein found in humanplasma at concentrations of 5-20ng/ml. In plasma, in in vitro studies,PF4 has been demonstrated to reverse the effect of heparin on clotformation. Michalski, Brit. J. Haematol, 38, 561-571 (1978).

In vivo experiments demonstrate that PF4 is cleared rapidly fromcirculation in both rats and rabbits. This rapid clearance is due to PF4binding to endothelial cells. PF4 bound by endothelial cell surfaces canbe released into the bloodstream by subsequent administration ofheparin. In humans, a similar rise in circulating PF4 concentration isobserved following heparin administration.

Platelet concentrate has been shown to have heparin neutralizingactivity when administered to humans after bypass surgery, and theeffect was attributed to PF4. Walker, Br. Heart J. 52:12-14 (1984).

SUMMARY OF THE INVENTION

The invention features a method for the neutralization of heparin in amammal having an undesirably high level of circulating heparin caused byadministration of heparin in connection with a medical procedure such asa surgical or therapeutic procedure, by administering a heparinneutralizing amount of purified PF4 or heparin binding fragmentsthereof.

Heparin binding fragments are those containing the heparin bindingregion, amino acids 61 to 66.

The term "purified" as used herein refers to PF4 or recombinant PF4(rPF4) which is 95% or greater, by weight, PF4 or rPF4, i.e., issubstantially free of other proteins, lipids, and carbohydrates withwhich it is naturally associated.

A purified protein preparation will generally yield a single major bandon a polyacrylamide gel for each subunit of PF4 or rPF4. Mostpreferably, the purified PF4 or rPF4 used in the compositions of theinvention is pure as judged by amino terminal amino acid sequenceanalysis.

We have discovered that normal coagulation status in mammals treatedwith heparin may be restored after administration of purified PF4without substantial depletion of platelets or leukocytes or otherdeleterious effects often associated with the commonly used heparinneutralizing agent, protamine.

In various aspects of the invention: The PF4 or fragment is rPF4 or afragment thereof. The PF4 or fragment is synthetic PF4 or a fragmentthereof. The heparin binding fragment of PF4 contains amino acids 61 to66. The fragment is C-41. The heparin neutralizing amount is sufficientto neutralize heparin administered at more than 50 units/kg of bodyweight. The amount is about 0.1 to 20 mg/kg of body weight. The amountis about 6 mg/kg of body weight.

The invention also features a method of restoring a normal coagulationstatus in a human patient after administration of heparin, withoutsubstantial depletion of platelets or leukocytes, by administeringpurified PF4 or a heparin binding fragment thereof to the patient toform a complex of the heparin with the PF4 or fragment.

The invention also features a method of treating a human patient duringa medical procedure, by administering to the patient a therapeuticallyeffective amount of heparin to inhibit coagulation, and thereafter,administering to the patient a therapeutically effective amount ofpurified PF4 or heparin-binding fragment thereof, to neutralize theheparin.

In various aspects of the invention: The medical procedure involvesheart surgery. The medical procedure requires a cardiopulmonary bypass.The medical procedure is an organ transplantation. The medical procedureis a kidney dialysis. The medical procedure is a catheterization. Thecatheterization is cardiac catheterization. The catheterization is forangioplasty. The medical procedure is for prevention of blood clotsformation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

We first briefly describe the drawings.

DRAWINGS

FIG. 1 is a graph showing platelet count, white blood cell count, andactivated partial thromboplastin time (APTT) in a rat treated withheparin, as a function of time after injection of protamine sulphate.

FIG. 2 is a graph showing platelet count, white blood cell count, andAPTT in a rat treated with heparin as a function of time after injectionof rPF4.

FIGS. 3 to 5 are bar graphs illustrating variations in white blood cellcount, platelet count, and APTT of rats treated with heparin, afteradministration of a control solution (FIG. 3), protamine sulfate (FIG.4), and rPF4 (FIG. 5). PF4

As is described in commonly owned U.S. Pat. applications, Ser. No.295,955, filed Jan. 10, 1989, and U.S. Pat. application Ser. No.451,021, filed Dec. 27, 1989, the entire contents of both applicationsbeing hereby incorporated by reference, human PF4 can be produced byrecombinant methods in E. coli. A synthetic gene was constructed basedon the known PF4 amino acid sequence, and a recombinant fusion proteinwas expressed which could be purified by heparin-agarose affinitychromatography after chemical cleavage of non-PF4 amino acids from therecombinant fusion protein.

The proteins are cleaved and extracted, and the extracts purified usingHeparin-agarose chromatography, removing contaminating proteins with0.6M NaCI and diluting with 1.2M NaCl. Eluted PF4 is dialyzed into 20 mMacetate buffer, pH 4.0, and then further purified by reverse phase HPLC.

As discussed in the above applications, heparin binding can beattributed to the region of the peptide containing residues 61 to 66. InVitro Experiments

In vitro experiments were conducted to compare the effects of protamineand PF4 for a preliminary assessment of

Platelet factor 4 is a well-known protein which has been completelysequenced (Deuel, T. F., R. M. Senior, D. Chang, G. L. Griffin, R. L.Heinrikson, and E. T. Kaiser [1981] Proc. Natl. Acad. Sci. U.S.A.78:4585-4587). It is a 70-residue secretable platelet protein with amolecular weight of approximately 7.8 Kd which is released duringplatelet aggregation. Although there is evidence of heparin bindingactivity and some indications of anti-angiogenesis activity (Folkman, J.[1984] Angiogenesis: Initiation and Modulation, Cancer Invasion andMetastasis: Biologic and Therapeutic Aspects, G. L. Nicholson and L.Milas, eds., Raven Press, N.Y., pp, 201-208), PF4 has never been shownto have clinical utility.

rPF4 Production

Recombinant PF4 was produced in E. coli as an N-terminal fusion proteincontaining a unique methionine residue immediately preceding the PF4portion. More specifically, expression plasmid pPF4-211 was constructedby cloning a synthetic gene encoding native sequence PF4 (see below)(Poncz et al., Blood, 69:219 (1987)) into the multiple restriction siteregion of plasmid pREV2.2 (deposited Jul. 30, 1986; accession #NRRLB-18091). Codon usage in the synthetic gene was optimized for expressionin E. coli, and synthetic DNA linkers were included on each end tofacilitate the directional insertion of the PF4 gene into the vector.The restriction sites HindIII and SmaI were chosen for insertion intopREV2.2. The resulting construct, pPF4-211, expressed a fusion proteincontaining 34 amino acids of E. coli β-glucuronidase (BG) separated fromthe PF4 sequence by a unique methionine residue. ##STR1##

Cells expressing the fusion protein were subjected to lysozyme (1 mg/gcells), DNase I (500 units/100 g cells) and bead mill treatments. Thelysis pellet containing the fusion protein was treated with CNBr (10g/100 g cells) in 70% formic acid to cleave the fusion protein at themethionine between the BG and PF4 portions. Following evaporation of theCNBr/formic acid, the recombinant protein was extracted with 200 ml of50 mM Tris-Cl, pH 7.6, 5 mM EDTA, and 10 mM DTT per 100 g of cellstarting material. Native sequence rPF4-211 was purified by binding theprotein to heparin agarose, removing contaminating proteins with 0.6MNaCl, and eluting with 1.2M NaCl. The resulting material was dialyzedinto 20 mM sodium acetate, pH 4.0, and analyzed on a 15% SDS-PA gelstained with Coomassie Brilliant Blue. Minor contaminants could beremoved using C₄ reverse phase high pressure liquid chromatography(HPLC) to prepare the protein for in vivo use.

PF4 peptides

Peptides were prepared by standard solid phase synthesis procedures,cleaved from the solid support and deblocked, and purified by reversephase HPLC.

Heparin Binding Fragments

The lysine rich region of PF4 (residues 61-66) is also the domainassociated with the binding of heparin by PF4. Heparin is known to playa role in modulating angiogenesis, which can also be affected byprotamine, another well characterized heparin-binding protein. To assessthe ability of PF4-based synthetic peptides to bind heparin, we assayedthe activity of coagulation-cascade enzymes which are inhibited byheparin. The Factor Xa assay used here has previously been described inDenton et al., Biochem. J., 209:455-460 (1983). Protamine and plateletfactor 4 are able to prevent the heparin inhibition of thrombin andFactor Xa at approximately equimolar concentrations. The 41 amino acidC-terminal peptide of PF4 (C-41) prevented heparin inhibition lesseffectively, but the C-13 peptide was unable to prevent the inhibitionof thrombin even at concentrations ten times that of an effective levelof rPF4.

The proteins are cleaved and extracted, and the extracts purified usingHeparin-agarose chromatography, removing contaminating proteins with0.6M NaCl and diluting with 1.2M NaCl. Eluted PF4 is dialyzed into 20 mMacetate buffer, pH 4.0, and then further purified by reverse phase HPLC.

As discussed in the above applications, heparin binding can beattributed to the region of the peptide containing residues 61 to 66.

In Vitro Experiments

In vitro experiments were conducted to compare the effects of protamineand PF4 for a preliminary assessment of whether PF4 might be a suitablesubstitute. Additionally, human platelet-derived PF4 and rPF4 werecompared.

In all properties examined, human PF4 and rPF4 showed essentiallyidentical characteristics. PF4 isolated from platelets and recombinantE. coli cells were compared for their abilities to prevent heparininhibition of Factor Xa (essential for the production of thrombin) invitro. The two protein preparations showed identical capacity forneutralization of heparin, with 1 nmol of protein neutralizingapproximately 0.25nmol (0.1 unit) heparin in this assay.

The abilities of protamine and rPF4 to prevent heparin-inhibition of thecoagulation enzyme Factor Xa were compared. The details of the assay aredescribed by Denton et al, 1983, Biochem. J. 209:455. In the Factor Xaassay, 0.5 units of heparin inhibited enzymatic activity almost 90%. Atlow concentrations, both rPF4 and protamine partially restored Factor Xaactivity, but at concentrations greater than 1 μM, rPF4 was clearly moreeffective in preventing Factor Xa inhibition.

In experiments carried out under conditions similar to the Factor Xaexperiments, thrombin was inhibited 99% by 0.25 units of heparin. BothrPF4 and protamine completely prevented heparin inhibition at 1 μM andshowed no significant difference in their molar dose responses.

Neutralization of heparin in human plasma was also examined. Theactivated partial thromboplastin time (APTT) of normal human plasma wasincreased from its base level of approximately 30 seconds to 195 secondsby the addition of 1 unit/ml of heparin. When added during theincubation preceding the addition of the activated thromboplastin, rPF4and protamine were equally effective in restoring the APTT to itsheparin-free level.

Similar studies and results may be obtained using certain fragments ofPF4 or rPF4 that include the heparin binding region, residues 61-66. Forexample, C-41, a 41 amino acid C-terminal peptide of PF4 described inU.S. Pat. application Ser. No. 295,555, (now abandoned) incorporatedabove, demonstrated the ability to neutralize heparin in vitro using theFactor Xa assay.

In Vivo Experiments

In Vivo experiments were performed on rats to compare the heparinneutralizing activity of protamine and PF4 and to investigate the effectof PF4 on platelet and white blood cell count.

Sprague Dawley rats (225g) were anesthetized with pentobarbital, thetrachea was intubated, and the right femoral vein and carotid arterywere cannulated. The intravenous infusion of heparin was followed byinfusion of either protamine or rPF4. Arterial blood samples were takenprior to and immediately after heparin infusion and at several timepoints following the infusion of the neutralizing agent. Controls wereconducted with Ringer s solution in place of heparin and theexperimental agents. Samples were used for the following determinations:clotting time, platelet count, and white blood cell count.

Referring to FIGS. 1 and 2, data are shown from a typical experimentcomparing the platelet count, white blood cell count and the APTT inrats when injected with heparin followed by protamine sulfate (FIG. 1)and when injected with heparin followed by rPF4 (FIG. 2). In theseFIGS., heparin was injected at t=0 and either protamine sulfate or rPF4injected two minutes thereafter (t=2 minutes). The dosages used were:heparin 1 unit/g animal wt.; protamine sulfate 1 ug/g animal wt.; andPF4 7.5 ug/g animal wt.

In FIG. 1, the effect of protamine is shown to produce a decrease in theAPTT to approximately normal levels after about 10 minutes. There wasalso observed a significant decrease of white blood cell count andplatelet count after injection with protamine sulfate. Ten minutes afterinjection of heparin and eight minutes after injection of protaminesulfate, platelet count had fallen from over 800×10³ /μl (measured att=0) to less than 500×10³ /μl (measured at t=10 minutes). Similarly,white blood count fell from over 5×10³ /μl (t=0) to less than about3×10³ /μl (t=10 minutes). These rats also had respiratory problems.

Administration of rPF4 also effectively neutralized heparin, as shown byrestoration of normal APTT levels at t=10 minutes (ten minutes afterinjection of heparin and eight minutes after injection of rPF4; FIG. 2).Neutralization of heparin by rPF4 did not lead to the extensivedepletion of platelet and white blood cells as caused by protaminesulfate. Platelet count remained relatively stable falling only from theinitial value of 700×10³ /μl (t=0) to about 500×10³ /μl (t=10 minutes).White blood cell count remained relatively stable at about 4×10³ /μlthroughout the tests.

Experiments were also conducted to investigate hematocrit levels underconditions as described with respect to FIGS. 1 and 2. No change inhematocrit levels were observed when either PF4 or protamine were usedfor heparin neutralization.

In addition, three experiments were performed in which heparinized ratswere injected with low doses of rPF4 (150 μg/100g). In these rats theanticoagulant effect of heparin was partially neutralized and no effecton platelets and white blood cells was observed.

Referring now to FIGS. 3 to 5, statistical bar graph data are showncomparing APTT, platelet count, (PC) and white blood cell (WBC) count inrats injected with a control solution, Ringers solution (FIG. 3),injected with heparin followed by protamine sulfate (FIG. 4) andinjected with heparin followed by rPF4 (FIG. 5). The data includedcontrol rats, five rats injected with heparin followed by protaminesulfate and two rats injected with heparin followed by rPF4.

FIG. 3 shows APTT, PC and WBC in rats after injection of mammalianRinger's solution. Samples A were taken before injection; samples B, 2min after injection; samples C, 10 min after injection; and samples D,18 min after injection. Data represents mean values and S.E.M. from 5experiments.

FIG. 4 shows APTT, PC and WBC in rats after injection of heparin (100units/100g) followed by protamine sulfate (100μg/100g). Samples A weretaken before injection; samples B, 2 min after heparin injection;samples C, 10 min after heparin and 8 min after protamine sulfateinjection; and samples D, 18 min after heparin injection. Data representmean values and S.E.M. from experiments except that white blood cellsand platelets were counted only in 3 samples taken at 2 and at 18 minafter heparin injection. The data in FIG. 4 indicate a drop in plateletand white blood cell counts in samples after protamine sulfate injectionto heparinized rats. The decrease was statistically significant (p<0.01)as compared to samples obtained before heparin injection.

FIG. 5 shows APTT, PC, and WBC in rats after injection of heparinfollowed by rPF4 (7.5 μg/g). Data represent mean values and S.E.M. from2 experiments for samples taken at times A-D as indicated in thediscussion above with respect to FIG. 4. The injection of rPF4 did notproduce a statistical decrease in platelet and white blood count.

As evident in the comparison of FIGS. 3, 4 and 5, platelet count remainsrelatively constant when rPF4 is administered to neutralize heparin(FIG. 5) compared to when protamine is administered to neutralizeheparin (FIG. 4). Statistically, platelet count decreased significantlyin the presence of protamine (48±12.2% of initial) but remains normalfollowing rPF4 (89±4.9%). WBC count also decreased following protaminesulfate treatment. These results indicate that PF4 is superior toprotamine in heparin neutralization in a number of respects.

USE

Purified rPF4 is prepared by a rigorously controlled procedure andsubstantially free of contaminants.

PF4 can be administered intravenously following heparin treatment, forexample, using a slow drip i.v. infusion over the course of 10 minutes.PF4 may be employed following heparin treatment in any surgicalprocedure in which heparin is employed or therapeutically when heparinis used to avoid clotting. A typical effective dose of rPF4 isapproximately 6.35 mg per kg of body weight, based on an effectiveprotamine dose of approximately 3.6 mg per kg of body weight. However,the actual dosage may vary with the dosage of heparin.

Other embodiments are within the following claims.

We claim:
 1. A method for completely preventing heparin inhibition ofcoagulation in a mammal having an undesirably high level of circulatingheparin following administration of heparin in connection with a medicalprocedure, said method comprising administering to said mammal a heparinneutralizing amount of purified PF4 or purified recombinant PF4.
 2. Themethod of claim 1 wherein said PF4 is recombinant PF4.
 3. The method ofclaim 1 wherein said PF4 is synthetic PF4.
 4. The method of claim 1wherein said PF4 comprises a heparin binding fragment, said fragmentcomprising the heparin binding region of said PF4 corresponding to aminoacids 61 to
 66. 5. The method of claim 4 wherein said fragment is C-41.6. The method of claim 1 wherein said amount is sufficient to completelyprevent heparin inhibition of coagulation when said heparin isadministered at more than 50 units/kg of body weight of the mammal. 7.The method of claim 1 wherein said amount is about 6.0 to 20 mg/kg ofbody weight of the mammal.
 8. The method of claim 7 wherein said amountis about 6 mg/kg of body weight.
 9. A method of restoring a normalcoagulation status in a human patient after administration of heparin,without substantial depletion of platelets or leukocytes, comprisingadministering purified PF4 or purified recombinant PF4 to said patientto form a complex of said heparin with said PF4.
 10. A method oftreating a human patient to temporarily prevent blood coagulation duringa medical procedure, comprising:administering to said patient atherapeutically effective amount of heparin to inhibit coagulation, andthereafter administering to said patient a therapeutically effectiveamount of purified PF4 or purified recombinant PF4 to neutralize saidheparin.
 11. The method of claim 10 wherein said medical procedure isheart surgery.
 12. The method of claim 10 wherein said medical procedureis a cardiopulmonary bypass.
 13. The method of claim 10 wherein saidmedical procedure is an organ transplantation.
 14. The method of claim10 wherein said medical procedure is a kidney dialysis.
 15. The methodof claim 10 wherein said medical procedure is a catheterization.
 16. Themethod of claim 15 wherein said catheterization is cardiaccatheterization.
 17. The method of claim 15 wherein said catheterizationis diagnostic.
 18. The method of claim 15 wherein said catheterizationis for angioplasty.
 19. The method of claim 10 wherein said medicalprocedure is for prevention of blood clot formation.